Electrooptical system



Aug. 8, 1939. F. GRAY ELEGTROOPTICAL SYSTEM Filed Nov. 5, 1935 PatentedAug. 8, 1939 UNITED STATES PATENT OFFICE ELECTROOP'EICAL SYSTEMApplication November 5, 1935, Serial No. 48,299

15 Claims.

This invention relates to television and, more specifically, totelevision employing low speed scanning.

Television images are ordinarily televised in a period of time withinthe period of persistency of vision so that they may be viewed withoutany flicker eiect being present. It is not always desirable to scan atthis speed as a slower rate of scanning reduces the frequency rangerequired for transmission.

One method o f producing images from currents produced by scanning theobject at low speed comprises producing them on a screen which continues to glow after being exposed to an exciting illumination or beamas, for example, a phosphorescent screen. Such images, however, arenormally produced with an objectionable amount of flicker. Thebrightness of an elemental area of the image decays rapidly after beingproduced 20 on the phosphorescent screen, and flashes up again at eachreproduction. The relatively low frequency of flashing results in anobjectionable amount of flicker.

An important object of this invention is to pro- 25 vide a novel methodof and means for reducing this iiicker which results from scanning animage in a period of time greater than the period of persistency ofvision.

Other objects and the various aspects of the invention will be apparentfrom the following description and the appended claims.

The brightness of the phosphorescence of an elemental area begins todecrease the instant the excitation ceases. The decay is of such anature that the fractional decrease in brightness is substantiallyindependent of the initial intensity of phosphorescence. If Bc is thephosphorescent brightness of the elemental area the instant theexcitation ceases, and B is the brightness at any time t after theexcitation ceases, then the ratio especially if a slow scanning speed isused. It has been discovered, however, that this flicker may becompensated for by inserting in front of the area a screen moving at therate at which the scanning proceeds in a direction transverse to theline scanning direction, the optical transmission of which varies withdistance along the screen in the inverse manner that varies with time.The apparent brightness of the image as transmitted through the screenwould thus be substantially constant. The correcting screen may be, forexample, in the form of a lm continuously moving across the eld of thephosphorescent screen.

The emission of light from an elemental area of the phosphorescentscreen within the short period of time that the exciting beam is passingover it may be so great that a special correction will be necessary. Oneway of accomplishing this is to modify the transmission at the edge ofthe section of the movable screen in order to maintain the apparentbrightness at a constant level during the interval in question. Anothermethod of correcting for this interval is by covering the screen fromthe view of the observer during the passage of the exciting beam as, forexample, by having narrow opaque strips at the edge of each section ofthe screen, the screen having zero transmission over these strips. Toprevent flicker, additional opaque strips may be included in eachsection so that any point in the eld is traversed by opaque strips in atime interval within the period of persistence of vision.

The invention will be more readily understood by referring to thefollowing description taken in connection with the accompanying drawing,in which:

Fig. 1 is a diagrammatic view of a television system utilizing theinvention;

Fig. 2 is a graphical representation of the decay of brightness withtime of a phosphorescent screen;

Fig. 3 shows the characteristics of a compensating screen;

Fig. 4 represents the apparent brightness of the phosphorescent screenas seen through the optical correcting screen;

Fig. 5 is an end view of the receiving tube showing its relation to thecompensating screen;

Fig. 6 shows a strip of lm for correcting for the initial increase inbrightness of the elemental areas of the phosphorescent screen; and

Fig. '7 is a graphical representation of the optical transmission withrespect to time of lm shown in Fig. 5.

Referring more particularly to the drawing, Fig. 1 shows a televisionsystem comprising a transmitting station ID, a suitable wire or radiochannel i i, and a receiving station R. Image currents modulated inaccordance with the tone values of successively scanned elemental areasof a eld of view are produced at the transmitting station i andtransmitted over the wire orV radio channel l l to the receiving stationR which has been represented, by way of example, as including a cathoderay receivingdevice l2. cathode ray receiving device comprises a cathodei3, a modulating cylinder or grid lli, an anode l5, two sets ofelectrostatic deilecting plates i6 and il and a phosphorescent screen i8. The incoming image currents are received, and ampliiedl if necessary,at the receiver i9 and applied between the'cathode I3 Yand themodulating cylinder or .grid id thereby modulating the cathode ray beamwhich. is caused to scan every elemental area of the phosphorescentscreen i8 by the wellknown action of the electrostatic elds between theplates I6 and I1.

v In order to produce the frequency range required for transmission, theobject at the transmitter is preferably scanned at a lower rate of speedthan that normally used, i. e., the object is scanned in a period oftime greater than the period of persistency of vision. Accordingly,therefore, when the phosphorescent screen i8 is viewed 'by the observerat the receiving station, there is present an objectionable amount offlicker. This may be compensated for by inserting in front of the screeni8 a correcting screen 20 moving at the rate at which the scanningproceeds in a direction transverse to the line scanning direction. Thiscorrecting screen 20 has, for purposes of illustration, been shown as anendless lm 2S moving between two sets of rollers 2i and 22. Fig. 5 showsthe end of the cathode ray tube l2 as viewed through the correcting orcompensating screen 28. Y

In order to understand the purpose and operation of the correctingscreen 2i), reference will now be made to Figs. 2, 3 and 4. Thephosphorescent screen i 8, which may be of any suitable material, suchas Zinc sulphide, continues to glow after being exposed to an excitingillumination or beam. The brightness of phosphorescence begins todecrease the instant the excitation ceases. This decay is of such anature that the fractional decrease of brightness is substantiallyindependent of the initial intensity of phosphorescence. Fig. 2represents graphically the decay of brightness Y Y of the excitedelemental area with respect to timeV Where a is a constant. The actualdecay of the screen may be measured and a correctingscreen constructedto have the transmission characteristics graphically represented in Fig.3 where T represents the optical transmission through the screen.

The

Suppose that a correcting screen were placed in front of thephosphorescent screen I8 and that the transmission T of the screenvaries with time in such a manner that T: ci? (l) as shown in Fig. 3,the constant c being less than one. Then the apparent brightness Ba ofthe area would be Ba=Bt *B B0) CB :CBD

In other words, the apparent brightness will be proportional to theproduct of the curves of Fig. 2 and Fig. 3 and therefore for a stillsubject independent of time as shown in Fig. 4. Consequently, therewouldbe no flicker in the reproduced eld. The brightness would, however, beproportional to the intensity of the exciting beam, a necessarycondition for the reproduction of an image.V Y

In most cases it is impractical to correct an entire image field in themanner just described, but substantially the same result may be attainedby moving a suitable transmission screen across the image eld. As anexample, take the case of an image reproduced on a phosphorescent screenin a cathode ray tube as shown in Fig. 1. The cathode beam sweepsrepeatedly across the screen il in a horizontal direction to reproducethe image strip, and moves slowly down the screen as the successivestrips are reproduced. For all practical purposes the reduction offlicker may be carried out as if all parts of an elemental strip werereproduced in the same instant, simply neglecting the short period ofline scanning in comparison to the much longer period required toreproduce an entire image.

The flicker may be reduced by a transmission screen inthe form of Yacontinuous belt moving down across the eld as shown in Fig. 1. This beltis. divided into sections of length Z just equal to the height of theimage eld. The optical transmission of any one of the sections increasesfrom the bottom edge to the top edge in the proper manner to eliminateflicker. For this purpose the optical transmission should vary withdistance along the sections in the inverse manner that varies with time.

. This condition may be expressed more exactly in mathematical terms asfollows: The ratio Then, if :c is the distance measured up from thebottom of the section, the transmission of the section varies along itslength so that T--KT Ft) where K is a constant and 'u is the Velocity ofthe screen. The transmission varies periodically in this manner overeach section along the belt.

The motion is synchronized with the cathode beam so that the edge of thesection moves down across the eld with the beam as it reproduces theimage. Ihis synchronization may be done in any well-known manner.

Considering any one strip of the image, the transmission of the screenin front of the strip at any time t after the passages of the beam isfound by substituting x=vt (5) in Equation (4). This gives K T- F-(t (6)The transmission of the screen in front of the elemental line thusvaries with time in the manner already found desirable above. Neglectingthe short period of line scanning, the instantaneous brightness of anelemental area in the strip is B=B0F(t) (7) The apparent brightnessthrough the screen is Ba=BT K =BUF(1)) (8) :KBO

For a still subject, the apparent brightness is thus independent oftime, and there will be no 'flicker in the reproduced eld. The sametheory 'optical transmission at the edge of a section may be modified inthe proper manner to maintain the apparent brightness of the constantlevel during the interval in question. The required trans- 'mission maybe .determined from measurements of brightness as a function of time. vAnother method of correcting for this interval is to cover the screenfrom the View of the observer during the passage of the exciting beam.This may be done by a narrow opaque strip 24 at the edge of each sectionof the belt, as illustrated in Fig. 6. The belt has zero transmissionover this strip. Such strips passing slowly across the eld would, inthemselves, introduce flicker. To prevent such flicker, additionalstrips 23 are included in each section, so that any point in the eld istraversed by opaque strips separated from each other by a time intervalless than the period of persistency of vision. For example, assumingthat images are reproduced at a rate of ten per second, an additionalstrip 23 may be included in the middle of each section as shown 'in Fig.6 so that any point in the eld is. traversed by opaque strips at therate of twenty per second. If desired, the screen may be mounted on arotating cylinder or disc and carried across the eld in that manner orany other suitable method for moving the screen may be employed.

Fig. 'l' discloses graphically the optical transmission through the beltshown in Fig. 6. It may thus be seen from this figure that the opticaltransmission of the belt is zero every time the opaque strips 23 or 24pass in front of the phosphorescent screen I8.

Instead of `designing the transmission screens from measurements of thedecay of brightness, they may be made photographically. For this purposea strip of phosphorescent screen may be instantaneously excited and thenphotographed on a moving lm. After proper development, the lm itselfserves as a correcting screen or it may be used to make correctingscreens.

While this invention has been described in connection with a beam ofcathode rays traveling over a phosphorescent screen on the end of acathode ray tube l2, it is obvious that the invention is not limitedthereto as it may be applied as well to any moving beam of radiationsprojected on a phosphorescent screen. If desired, the cathode ray tubeI2 and its accompanying correcting screen 20 may be located in aprojection booth and the image corrected for ilicker projected onto ascreen.

The invention is not narrowly limited to use with phosphorescent screensas it may be utilized with any apparatus producing an image whosebrightness decays with time after van element of the image has beenproduced. For example, it may be used in conjunction with a bank of hotfilament lamps or with a bank of light valves closing slowly with time.In the latter case, it might be more convenient to locate the movingscreen sections in a projection apparatus instead of moving them acrossthe large screen on which the image is projected. Various othermodifications may obviously be made without departing from the spirit ofthe invention, the scope of which is dened by the appended claims.

What is claimed is:

l. Image producing means comprising means for emitting light at anyelemental portion of a iield when impulsively energized in that portion,which light thereafter slowly decreases in intensity in a predeterminedmanner, means for causing image current representative of light tonevalues of successively scanned elemental areas of an object or field ofview to control the impulsive energization of said light emitting meansfor each elemental portion of said eld in turn and repeatedly at such arate that said alternate increases and decreases in light intensitybecome apparent as a flicker to the eyes of an observer upon` which saidlight imp-inges, a-nd means through which said light passes and which isplaced between said light emitting means and said observer for reducingsaid nicher, said lastmentioned means comprising a light transmittingscreen, the transparency of which varies along its length.

2. Image producing means comprising means for emitting light at anyelemental portion of a iield when impulsively energized in that portion,which light thereafter slowly decreases in intensity in a predeterminedmanner, means for causing image current representative of light tonevalues of successively scanned elemental areas of an object or field ofview to control the impulsive cnergization of said light emitting meansfor each elemental portion of said field in turn and repeatedly at sucha rate that said alternate increases and decreases in light intensitybecome apparent as a flicker to the eyes of an observer upon which saidlight impinges, and means through which said light passes and which isplaced between said light emitting means and said observer for reducingsaid ilicker, said last-mentioned means comprising an optical correctingscreen and means for causing said screen to move across the eld of Viewof said light emitting means,

said screen Vhaving light transmission) characteristics which compensatefor the decrease in light intensity of the light emitting means withrespect to time.

3. Image producing means comprising means for emitting light at anyelemental portion of a field when impulsively energized in that portion,which light thereafter slowly decreases in intensity in a predeterminedmanner, meansk for causing image current representative of light tonevalues of successively scanned elemental areas of anY object or fieldlof view to control the impulsive energization of said light emittingmeans for each elemental portion of said eld in turn and repeatedly atsuch a rate that such alternate increases and decreases in lightintensity become apparent as a flicker to the eyes of an observer uponwhich said light impinges, and a moving film through which said lightpasses and which is placed between said light emitting means and saidobserver for decreasing the higher values of light intensity wherebysaid iiicker is reduced, said film having opaquer strips uniformlyplaced thereon.

4. Image producing means comprising means for emitting light atanyelemental portion of a eld when impulsively energized in that portion,which light thereafter slowly decreases in intensity in a predeterminedmanner, means for causing image current representative of light toneValues of successively scanned elemental areas of an object or field ofView to control the impulsive energization of said light emitting meansfor each elemental portion of said field in turn and repeatedly at sucha rate that such alternate increases and decreases in light intensitybecome apparent as a flicker to the eyes of an observer upon which saidlight impinges, and means through which said light passes and which isplaced between said light emitting means and said observer fordecreasing the higher values of light intensity whereby said flicker isreduced, said last-mentioned means comprising a moving lighttransmitting lm having opaque strips uniformly placed thereon, saidstrips adapted to cut ofi" from the eyes of the observer the high valuesof light intensity as they are produced, and additional opaque stripslocated between said rstmentioned strips for preventing saidfirst-mentioned strips from causing a iiicker eiect.

5, Image producing means comprising means for emitting light at anyelemental portion of a field when impulsively energized in that portion,which light thereafter slowly decreases in intensity in a predeterminedmanner, means for causing image current representative of light tonevalues of successively scanned elemental areas of an object or eld ofview to control the impulsive energization of said light emitting meansfor each elemental portion of said field in turn at such a low speedthat said alternate increases and decreases in light intensity becomeapparent to an observer as a flicker, and means at said elementalportions acting in a manner complemental to said decrease in intensitywhereby the intensity of light reaching the observer is substantiallyuniform with respect to time for at least a large part of the time thatlight is emitted.

6. Image producing means comprising means for emitting light at anyelemental portion of a field when impulsively energized in that portion,which light thereafter slowly decreases in intensity in a predeterminedmanner, means for causing image current representative of light toneValues of successively scanned elemental areas of an object or iield ofview to control the impulsive energization of said light emitting meansfor each elemental portion of said eld in turn at such a low speed thatsaid alternate increases and decreases in light intensity becomeapparent to an observer as a iiicker, and means at said elementalportion acting in a manner complemental to said decrease in intensitywhereby the intensity of light reaching the observer is substantiallyuniform with respect to time for at least a large part of the time thatlight is emitted, said last-mentioned means comprising a screen, theoptical transmission of which varies along its length.

'7. The combination with means for setting up transient imagescomprising an array of light emitting elements which when activated emitlight with decaying intensity for a time after activation, means foractivating elemental portions of said array in succession and at a ratesuch that all of said elements are activated in a period much greaterthan that of persistence of vision, and a moving film having opaqueportions thereon for interrupting the light reaching an observer fromeach of said elemental portions for relatively brief intervals at aperiodicity which is an integral fraction of that required to set up acomplete image.

8. Light emitting means, means for causing said light emitting means tointermittently emit light, the intensity of which varies during eachperiod of emission in a predetermined manner, and means interposed inthe path of said light on its way to an observer, the eiective opticaltransmissibility of which last-mentioned means varies in a mannercomplemental to said variation in intensity throughout each period oflight emission.

9. Light emitting means, means for causing said light emitting means tointermittently emit light, the intensity of which varies during eachperiod of emission in a predetermined manner, optical means interposedin the path of said light on its way to an observer to cause theintensity of said light leaving said optical means to be substantiallyuniform throughout any period of emission, the effective opticaltransmissibility of which last-mentioned means varies in a mannercomplemental to said variation in intensity throughout each period oflight emission, and means for varying the intensity of light emittedfrom said light emitting means in accordance with signals.

10. Image producing apparatus comprising a plurality of light emittingelements, means under control of incoming image current for controllingsaid elements to `cause them to emit light in succession to producelight tone values representative of the object, an image of which is tobe formed, said light from each element varying in intensity during eachperiod of emission, and means interposed in the path of light from saidelements on its way to an observer to cause the intensity of said lightleaving said means to be substantially uniform throughout any givenperiod of light emission, said last-mentioned means comprising a movinglm, the transparency of which varies over its surface.

11. Image producing means comprising means under control of incomingimage current for Causing light to be emitted from each elemental areaof the image eld in succession and to persist at each elemental areaafter control by the incoming image current ceases, and a moving film,the transparency of which varies over its surface interposed in the pathof light from said light emitting means on its way to an observer forperiodically obscuring said light.

12. Image producing means comprising means under control of incomingimage current for causing light to be emitted from each elemental areaof the image eld in succession and to persist at each elemental areaafter control by the incoming image current ceases, and means includinga film, the transparency of which varies over its surface inter-posed inthe path of the light from said image field to a position Where it maybe utilized for controlling the period during Which said emitted lightis effective.

13. Image producing means comprising means under control of incomingimage current for causing light to be emitted from each elemental areaof the image field in succession and to persist at each elemental areaafter control by the incoming image current ceases, and means includinga iilm, the transparency of which varies over its surface interposed inthe path of light from said light emitting means for controlling theperiod of persistence of the eiective light from said light emittingmeans and also for variably modifying said light as a function of time.

14. Television image producing apparatus comprising a member formed ofelements which are capable of emitting light for a period after initialacceleration from without takes place and the effective light emittingportions of which form an array of elements in straight parallel lines,of means for initiating the emission of light from said elemental lightemitting portions progressively along parallel lines thereof insuccession at a rate such that the emission period of each elementoverlaps those of a large number of other elements, a member havingparallel straight lightobstructing portions interspersed with portionsof relatively great light-transmissibility, and means for moving saidmember so that said portions cross the paths of light from said elementsto cause said opaque portions to become effective in succession tointercept light from one elemental line after another of said lightemitting member at the rate at which said lines are energized insuccession.

l5. Television image producing apparatus comprising a member formed ofelements which are capable of emitting light for a period after initialacceleration from Without takes place and the effective light emittingportions of which form an array of elements in straight parallel lines,of means for initiating the emission of light from said elemental lightemitting portions progressively along parallel lines thereof insuccession at a rate such that the emission period of each elementoverlaps those of a large number of other elements, a member havingparallel straight lightobstructing portions interspersed with anddecreasing in light-obstructive power in a predetermined manner toportions of relatively great light-transmissibility, and means formoving said member so that said portions cross the paths of light fromsaid elements to cause said opaque portions to become effective insuccession to intercept light from one elemental line after another ofsaid light emitting member at the rate at which said lines are energizedin succession.

FRANK GRAY.

